Allied Telesis AT-S62 User Manual
Page 193
AT-S62 User’s Guide
Section II: Advanced Operations
193
Each switch port has four egress queues. The queues are Q0, Q1, Q2, and
Q3. Q0 is the lowest priority queue and Q3 is the highest. A packet in a
high priority egress queue is typically transmitted out a port sooner than
a packet in a low priority queue.
Table 8 lists the mappings between the eight CoS priority levels and the
four egress queues of a switch port.
For example, assume that a tagged packet with a priority level of 3
enters a port on the switch. The switch, after examining the packet’s
destination address, determines that the packet is to be sent out port 6.
The switch must now determine in which of port 6’s egress queues the
packet should be stored. It examines the priority level in the packet,
which is 3. Now the switch knows to store the packet in port 6’s Q1
egress queue.
You can change these mappings. For example, you might decide that
packets with a priority of 5 need to be handled by egress queue Q3 and
packets with a priority of 2 should be handled in Q0. The result is shown
in Table 9.
The procedure for changing the default mappings is found in Mapping
CoS Priorities to Egress Queues on page 200. Note that because all ports
must use the same priority-to-egress queue mappings, these mappings
are applied at the switch level. They cannot be set on a per-port basis.
Table 8 Default Mappings of IEEE 802.1p Priority Levels to Priority Queues
IEEE 802.1p Priority
Level
Port Priority Queue
0 or 1
Q0 (lowest)
2 or 3
Q1
4 or 5
Q2
6 or 7
Q3 (highest)
Table 9 Example of Customized CoS Mappings to Priority Queues
IEEE 802.1p Priority
Level
AT-S62 Priority Queue
0, 1, or 2
Q0 (lowest)
3
Q1
4
Q2
5, 6, or 7
Q3 (highest)